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Thursday, March 17, 2011

Status update: Fukushima Daiichi Reactor Plants

While there seems to be a lot of down time regarding news from Fukushima Daiichi -- at least in terms of hard news, because there's lots and lots of panic, hyperbole, supposition, and error filling the big media (even including runs on iodine pills in the US and supposed orders for all US citizens to evacuate Japan) -- it might be a good time to step back a moment and take a look at what the condition of the reactor plants there really seems to be at the moment, and what TEPCO, and the Japanese government are really focusing on.

THE BASICS: Reactor safety is, as always, paramount -- and that extends to situations even after there is core damage known or suspected! Thus, it's important for TEPCO to get all of the affected nuclear reactors in a safe, stable condition. What does this mean? It means that the reactor cores must be ...

-Ensured shut down; by use of control rods, and boron in seawater being used to flood the reactor pressure vessels-Cooled; being done by seawater since the backup systems cannot presently be used either because of loss of electricity or loss of system integrity or loss of remote operability-Monitored; being done by many inferred methods at this time, as well as possibly some of the original methods

What is the problem I hint at above with monitoring? Well, let's look at something that might be failed -- like the ability to detect what the water level in the core, which is inside the pressure vessel, really is. Normally operators in the control room are looking at meters; these meters are deriving a signal from equipment located nearby which is receiving a signal from a differential pressure cell located beside the reactor pressure vessel and which is connected to the equipment by wires. So you have reactor, detector, wires, equipment, more wires to control panel and finally a meter.

None of this might be working right now. So what do you do? Well, if you have a pump hooked up to the reactor at the base, say, and you have a discharge pressure gauge you can figure out how high of a column of water that pump can provide by looking at this pressure. A head of water measurement is possible to infer this way. Or, if the pressure suddenly goes waaay up rapidly the plant is "solid" and there is no steam or gas volume left. This very simplified example is just one of many many that surely is occurring at Fukushima Daiichi right now as the operators first struggle to find out what is going on inside destroyed buildings with deranged equipment and little control room indication.

And then there's remotely OPERATED equipment! Control switches in the control room operate valves and pumps located right at the reactor. If these are failed, other ways have to be found to get what you want done, done. Need to vent pressure? There might have to be another way improvised than just operating a switch and watching a meter showing pressure. Again, this is very simplified but this is just a hint of what's going on.

And all this in a high radiation field, with radioactive contamination on various surfaces all over, and radioactive gases in the air at times.

THE REACTORS: It's safe to say that all reactors that were critical at the time of the quake have sustained core damage. In varying degrees. Probably all have some degree of secondary containment damage, but only one appears to have problems with its PRIMARY coolant system integrity. And this is still uncertain.

Seawater has had to be used to cool the reactors further after some core damage, which ruins them for good. Boric acid was added to ensure subcriticality (boron, a component of boric acid, absorbs neutrons; if you absorb them in boron they can't get absorbed in Uranium and cause fissions.) The cores, and the drywells outside of them, appear to have had seawater admitted to all plants at some point.

One positive aspect --if there is any -- is that while the whole problem with the spent fuel pools has been going on, little about the actual nuclear reactors has come out and frankly the JAIF status reports are fairly stable, with the exception of continuing I&C (instrumentation and control) equipment failures.

WHAT NOW? Still have to rapidly get I&C back (and that's part of why TEPCO is getting high voltage lines strung in brand new) to allow detection and operation, and also to operate those remotely operated pumps and valves... The point is to try to get some semblance of normalcy regarding control and monitoring to make the situation workable. The spent fuel pool condition will improve too, but only if the cooling systems for these are intact (and we find it hard to imagine that the system in No. 3 is intact.)

This is one good reason NOT to store spent fuel at reactor sites, but rather transfer it offsite, as I've been promoting without luck for years. Just an aside.

So this, coupled with the previous JAIF status report (and I encourage everyone to read this and get familiar with some of the terms, and look back through this blog's illustrations and see what is what inside a BWR plant like these are) should give a good status check so that further information from here forward is less confusing.

Expect more water operations tonight, both attempts to cool the fuel pools and probably concerted attempts to get No. 2 plant's water level back up in the reactor pressure vessel.

2 comments:

Regarding the reactors, I was under the impression that all of the reactors shut down at the time of the earthquake, and thus none would have been critical at the time of the tsunami.

A perhaps 'silly' question: if the reactors had not automatically shut down at the time of the earthquake, could they have powered their own cooling systems after the tsunami? I guess this would also require that the generators be able to function in 'island' mode, and that the reactor not shut down in response to the load dump when the lines were lost.

Yes, and I realized my error and reedited the story right after I first published it. The reactors full scrammed at the time of the quake, and then one hour or so elapsed before the tsunami hit. No way you'd want the reactors still critical when the tsunami hit, which is exactly why they scram out when the big quakes happen. HOWEVER, it is beginning to look like there will be a host of changes to the coastal plants after this is all over vis a vis auxiliary site power, emergency offsite power provision and protection of equipment.